Computer modelling of the polymerization of pyrenyl actin demonstrates that the effects of profilin on polymerization can be explained entirely by the formation of a 1:1 complex between profilin and actin monomer with a KD of 3-5 MuM. The only effect of the presence of profilin is to reduce the concentration of actin monomer available for polymerization so that the rates of nucleation and polymerization are affected as predicted by the model. Because the concentration of the complex is a function of the concentration of actin monomer, the presence of profilin serves to amplify greatly the changes in concentration of F-actin at steady state that result from anything that changes the critical concentration of actin monomer. Therefore, the physiological roles of profilin mayt be to minimize the effects of changes in monomer concentration on the rate of actin polymerization and amplify the effect in the concentration of F-actin at steady state. Simultanous analysis of the kinetics of exchange of actin monomer and polymer at steady state and the kinetics of ATP hydrolysis lead to the proposal that actin filaments contain a cap of actin.ATP subunits although the bulk of the filament contains actin.ADP subunits. AMP and ADP competitively inhibit ATP hydrolysis at steady state but do not affect actin exchange at steady state of ATP hydrolysis during pre-steady state polymerization. The complete amino acid sequence of Acanthamoeba actin has been determined. Its most unusual feature is the presence of N-terminal glycine.